A-Scan Technique
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A-scan technique
Fundamentals
The A-scan is the basic presentation of results for every ultrasound investigation and is always used for evaluation in the event of problematic measurement results. The A-scan is characterized by the fact that the amplitude of the HF-scan is formed (Fig. 1). This means that the negative half-waves are “flipped” upwards, while their amplitudes are retained. This makes it much easier to determine the attenuation of the ultrasonic signal by comparing amplitudes directly on the screen of the ultrasonic device.
| Fig. 1: | Principle of A-scan imaging
(S – transmitted signal, F – failure echo, R – back-wall echo, d – test piece thickness) |
Doubling the amplitude height on the screen by shifting the 0 dB line to the lower or upper edge of the image is an important visual advantage for the inspector and allows for a more accurate evaluation of the amplitudes during defectoscopy or wall thickness measurement. The advantages of the A-scan can be seen by comparing it with the HF-scan. However, phase jumps are no longer visible, which may lead to misinterpretation of the measurement signal.
Example of an A-scan for polyester resin
Many A-scan devices offer the option of reading out the curve values (blue in Fig. 2) using an oscilloscope and processing them further with suitable software. The analog curve is digitally recorded in its HF-form and stored on the measuring computer as a text file in ASCII format.
| Fig. 2: | Example of an A-scan from an ultrasonic measurement on a polyester resin with a wall thickness of 9 mm |
Another disadvantage of the A-scan is that the negative half-waves can no longer be reconstructed because the measurement curve is not continuous, and therefore a Fourier transform (see: frequency analysis) is no longer possible. In contrast, by reading out the digitized HF-scan using the ASCII data, the Fourier transform can be performed to display the frequency of the measurement signal.
See also
- B-scan technique
- Imaging ultrasonic testing
- C-scan technique
- D-scan technique
- F-scan technique
- HF-scan
- Ultrasound testing
References
- Grellmann, W., Seidler, S. (Eds:): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22)
- Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin Heidelberg (1997) (ISBN 3-540-62072-9; see AMK-Library under M 45)
- Matthies, K. u. a.: Dickenmessung mit Ultraschall. Deutsche Gesellschaft für Zerstörungsfreie Prüfung, 2nd revised Edition, DVS-Publishing (1998) (ISBN 3-87155-940-7; see AMK-Library under M 44)